Well
Also Romani hypothesizes Fate Zero and Fate GO are close to the center of the trunk in the Zero Collab.
For what you're saying to be true, Tsukihime Worlds would have to cluster on one edge of the trunk.
But proto-worlds are called 異世界 so they aren't on this tree or are designated for pruning.
Tbh tinfoil but since we known pruning and stuff happens because of Alaya and the human unconsciousness. What if in the Tsukihime Worlds because the Human Order is weak, pruning and QTLs just stopped happening at one point. And that lack of energy is the coming of the Sixth.